I carry several pieces of rubber shelf liner in my gig back. You can get the kind with holes in it or another version that is just like a rubberized mat.
I actually used mine yesterday when a wedding coordinator set me up to play with my harp half on an area rug and the other half on a slate floor. My pedal harp was definitely not steady! I just folded my rubber mats to the size, shape and thickness I needed and set my harp down with no problems.
Outside I bring a 3×6 foot piece of plywood (painted green for “grass” on one side and brown for “dirt” on the other side) for my pedal harp. It’s big enough for both me and my bench. For my Celtic harp, I just have a several pieces of Plexiglas (one is 12×18″, the other is 18×24″) to put under the harp. These would also work well indoors.
Hope one of these ideas helps.
Alice in windy Wyoming (where it is a very pleasant 73º right now)